The only place you may want to use a bipolar el cap (with a film bypass) is the output of the EQ sction. That can swing dc offsets positive or negative depending on frequency sweep positions. The rest can be polarized low impedance caps. Just position them with + dc facing the + pin. Don't use OPA134 except for the fader amp, it's a single. A LME49710 is better.

Very interesting to hear what you say about the eq output cap. I hadn't considered that.

Wrt the chips - my desk seems to use single chips (loads of various fairchilds, I think) rather than dual channel chips (like the spare channel I have from a later desk).

On mine there is a single TLO71 at the mic/line pre which I was thinking of swapping for the OPA134s. I have heard they are similar in sound only with better bass. I have also heard that those LMEs are very good chips. I have just ordered chip sockets for my input channels to allow me to try a few flavours.

Slight correction to the above - if I am reading the board/schematic properly it looks like I have one TL071 in the mic pre and the line pre has a dual UAF772TC, which is split with the low pass filter.

I might try a few OPA134s in the mic pres for different flavours but I'll probably keep the line pre chips all the same as each other for a consistent sound, once I have decided what to use (or even whether to change them.

With the LMW 497**s do I need local decoupling on the power rails? If so, what's a good value? Does anyone know the current draw on these chips?

Also (back on topic) if anyone has a copy of the Soundcraft 2400 grounding mod sheet I would be most grateful if I could have a look. So far I've drawn a blank on the net.

I have modded 8 channels completely, both recapping and cutting the earth trace.

Unfortunately, on my input cards the only eay to cut the trace was to cut a line around the input/output connectors separating them from the thick earth trace (using a dremmel tool). Gosh that hurt. I know it is in the interest of improving the desk's sound but it isn't enjoyable hacking vintage gear.

As I suspected, the improvement would only start becoming apparent after I had done a few channels. As you'd expect, the amount of hum that each individual channel adds is also dependent on the number of addition earth loops there (i.e. unmodded channels).

The difference is now noticable. I can't quite see why I only had a partially modded desk to start with. Odd.

I now have Burr Browns in the first 8 mic pres and I think I'll leave it at that - I'll leave TL071s in the rest. I haven't had a chance to critically compare these 2 chips yet.

I was wondering whether it might also be fun to wire in some transformers on a few mic pres. Does anyone have suggestions for good sounding but cost effective transformers for the job?

It appears our 2400 has also had the mod partially done like yours - with the star ground being installed and a copper bar running along under the channel strips joined to the channels by thick green wire. However, the original grounds from the ribbons were not cut. All of the channels were done but the patch bay was left untouched......was yours similar? How odd, I wonder why people did half the mod, and how many more desks out there are half modded?

Just thought it was interesting that someone else has encountered the same issue